1. Field of the Invention
The present invention relates to a blood pressure information measurement device and a method of calculating an arterial stiffness index with the blood pressure information measurement device. More particularly, the present invention relates to a blood pressure information measurement device that measures blood pressure information and effectively determines a degree of arterial stiffness, and a method of calculating an arterial stiffness index with the blood pressure information measurement device.
2. Description of the Related Art
Heretofore, as a device for determining the degree of arterial stiffness, JP 2000-316821A, for example, discloses a device that examines the propagation velocity of a pulse wave ejected from the heart (hereinafter, PWV).
By placing cuffs or the like for measuring the pulse waves at two or more locations such as the upper arm and the lower leg, for example, and measuring the pulse waves simultaneously, PWV is calculated from the difference in times of the pulse waves at the respective locations and the length of the artery between the two points at which the cuffs or the like for measuring the pulse waves are placed. Thus, at least two locations for placing cuffs or the like are needed, and it is difficult to easily measure PWV at home.
In view of this, as a device for determining the degree of arterial stiffness from the pulse wave measured at the location of the upper arm, JP 2004-113593A, for example, discloses a device provided with a cuff for pulse wave measurement and a compression cuff for compressing the distal side. The difference in times of an ejected wave and a reflected wave can be detected using not only the pressure pulse wave measured at the upper arm but also the pulse wave measured by tonometry at the carotid artery or the radial artery.
With the device of JP 2004-113593A, pulse wave velocity serving as an index for determining the degree of arterial stiffness is calculated by separating the ejected wave that is been ejected by the heart from the reflected wave that is reflected by the main reflection site in the aorta by measuring the pulse wave on the side on which the heart is located while compressing the distal side, and detecting the difference in appearance times of the ejected wave and the reflected wave (referred to as Δt, PTT, Tr, etc.). Specifically, the difference in respective propagation paths of the ejected wave and the reflected wave to the measurement site is converted to a pulse wave velocity (PWV), serving as an index for determining the degree of arterial stiffness, by being divided by the time difference between an appearance of the ejected wave and an appearance of the reflected wave at the measurement site.
In such a device, information on the pulse wave propagation distances is needed in addition to the pulse wave propagation times in order to accurately calculate the pulse wave velocity. If the propagation times are the same, this indicates a high degree of arterial stiffness, since propagation velocity increases as propagation distance increases.
Conventionally, the main reflection site of the pulse wave in the aorta is said to be the iliac artery bifurcation. In view of this, a distance (2L_da) obtained by doubling the distance (L_da) from the aortic root to the iliac artery bifurcation was assumed to be the difference in respective propagation paths of the ejected wave and the reflected wave to the measurement site, and a value (2L_da/Tr) obtained by dividing this difference by Tr (Traveling time to reflected wave) was considered to be equal to the PWV of the aorta.
However, the main reflection site is, in fact, not fixed but changes due to differing attributes of the person being measured, such as age and height, and the clinical condition of the person being measured, such as the extent of arterial stiffness. Thus, there is a problem in that an error occurs in the calculated PWV when the reflection site is assumed to be the same for all people.